A recent development in communications technology has been the introduction of synchronous networks commonly referred to as synchronous digital hierarchy (SDH) or, in North America, as SONET. Synchronous networks have the significant advantage of simplifying the process of demultiplexing channels and thus avoid the need for the so-called multiplexer/demultiplexer mountains that were associated with previous plesiochronous techniques.
Synchronous networks are now being used to carry both synchronous and asynchronous/ATM traffic. The asynchronous or ATM traffic is carried in cells which are carried over the synchronous network in standard SDH virtual containers (VCs). The cells are recovered from the virtual containers into the standard ATM transmission format at the egress from the synchronous network. Arrangements for carrying both synchronous and ATM traffic are described for example in U.S. Pat. No. 4,926,416 and IEE Conference Publication No 39, October 1990, pages 73-78.
These conventional systems suffer from the disadvantage that they are of complex construction and require a relative large number of ATM ports to handle the ATM traffic. Further, many of these systems are inefficient in their use of bandwidth.